This application is related to Japanese application No. HEI 11(1999)-264363 filed on Sep. 17, 1999, whose priority is claimed under 35 USC xc2xa7119, the disclosure of which is incorporated by reference in its entirety.
1. Field of the Invention
The present invention relates to a polycrystalline ferromagnetic metal oxide and a method of manufacturing the same. More particularly, it relates to a polycrystalline ferromagnetic metal oxide with an ordered perovskite structure having high density and low resistance, and a method of manufacturing the same.
2. Description of Related Art
Recently, magnetic heads having a recording density of 40 Gbit/inch2 has been targeted. For this purpose, considerable research has been done on materials or multi-layer films with effect such as GMR (giant magnetoresistance) and TMR (tunnel magnetoresistance) utilizing tunneling junction at ferromagnetic thin films sandwiching an extra-thin insulating layer.
However, the GMR materials exhibit magnetoresistance (MR) of less than 10%. Therefore further breakthrough is required for higher density.
In contrast, the TMR multilayer metal films utilizing an insulating layer of Al2O3 and the like exhibit MR of more than 10%, but advanced techniques with great precision are required for forming the multilayer film. For the purpose of higher recording density, it is expected that the magnetic head approaches or contacts a medium and is utilized under high temperature. Accordingly, characteristics of the TMR materials may possibly be deteriorated by element diffusion or the like, since the multilayer formation has been controlled at low temperature.
It is known that CMR (colossal magnetoresistance) materials exhibit full spin polarization at absolute zero, and thus they are expected to show high MR of almost 100%. Oxides thereof are not influenced by temperature and air, and deterioration of the characteristics due to the element diffusion is less serious in the temperature range of several hundred xc2x0 C. because of stability to temperature and air of the materials. However, none of them can give favorable characteristics at room temperature so far.
For an oxide of ordered perovskite structure Sr2FeMoO6, magnetoresistance has recently been observed at room temperature. This material has a high Curie temperature and thus expected to exhibit spin polarization of about 70% at the maximum even at room temperature. MR calculated from the spin polarization value is almost 100%, therefore the material is considered as a magnetic head material for future generation.
Kim et al. describe about electrical and magnetic characteristics of the grain boundaries of the material (Applied Physics Letters Vol. 74, No. 12, p1737-1739(1999)). Whether the grain boundaries are coupled ferromagnetically or antiferromagnetically depends on the material of ferromagnetic grains constituting the grain boundaries, even though the ferromagnetic grain boundaries are insulative. Ordered perovskite materials such as Sr2FeMoO6 belong to the former and manganese perovskite materials belong to the latter. The magnitude of the magnetoresistance highly depends on the temperature. Therefore as the temperature increases, the magnetoresistance of the former materials abruptly decreases, whereas that of the latter materials gradually decreases. In the ordered perovskite materials such as Sr2FeMoO6, this is advantageously applied to obtain great magnetoresistance at room temperature.
The TMR materials have been suffering from large resistance, and thus a material with a hybrid structure of a layered one and a granular one has been proposed. The ordered perovskite oxide Sr2FeMoO6 exhibits the resistivity of about 1 mxcexa9xc2x7cm or less at room temperature in the form of single crystal or single crystalline thin film, but the resistivity increases to 28.5 mxcexa9xc2x7cm in the form of a polycrystalline bulk. In addition, the thin film formation requires highly advanced techniques to grow atomic layers one by one to form the film, which increases the production costs.
The inventors of the present invention have noticed that the ordered perovskite structure comprises a three-dimensional network of a FeO6 octahedron and a MoO6 octahedron which contributes to the conductivity, and thus the electronic conductivity is highly three-dimensional. Further, since the carrier density is as high as 1022 atm/cm3, they have reached the conclusion that the ratio of carrier trap to the carrier can be suppressed, which can be effective by achieving by avoiding the generation of an insulating phase to be a tunnel barrier.
Then, intensive research has been conducted to densely couple the crystal grains in an atomic order. As a result, atomic interfaces of not less than 97% density without any heterogeneous phase in TEM level can be obtained with the aid of high-pressure treatment process, and then a polycrystalline substance having resistivity as low as 1 mxcexa9xc2x7cm or less can be produced. Thus, the present invention has been achieved.
According to the present invention, provided is a polycrystalline ferromagnetic metal oxide having a density in the range of 97% to 100%.
Still according to the present invention, provided is a method of manufacturing a polycrystalline ferromagnetic metal oxide by means of a treatment for high densification of a polycrystalline ferromagnetic metal oxide under high-pressure reducing gas.
These and other objects of the present application will become more readily apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.